In our patent application PCT/SE94/01166 we described a method of selection of antigenic or other specific binding protein molecules, and their encoding DNA, by means of a phage display package produced by co-expression of a phagemid vector and a helper phage, in which the DNA encoding the binding peptide is inserted into the phagemid vector so as to be expressed as a fusion with a truncated phage gene III protein. The resulting display package lacks functional protein III, and is therefore not infectious, but it can be selectively rendered infectious by means of a fusion protein comprising functional protein III and the complementary binding partner for the specific binding protein present on the surface of the phage display package. The function of the helper phage is to provide the genes necessary for packaging the expression products of the phagemid; but it will be deficient in gene III so that the resulting phage display package will not contain functional gene III.
A similar system has been disclosed also in example 2 of EP-A-614989. In that case, the helper phage used was a derivative of phage Fd tet, known as fKN16, which had a deletion of 507 nucleotides in gene III. In our PCT application we used a derivative of phage M13 KO7 having a 1121 nucleotide deletion in gene III between nucleotides 1525 and 2646.
We have found that this M13 helper phage is considerably better than phage fKN16 in the performance of the procedure. Not only that, we have now also found that it is similarly better than another Fd phage (fCA55) having a larger deletion in gene III; and furthermore, that we can improve on our M13 phage construction previously described.
The experimental basis for this is presented below. In summary, however, the following general features seem to emerge.